1. Field of the Invention
The present invention relates to a mobile radio terminal such as a portable telephone or the like for performing transmission/reception through a single antenna, or an antenna duplexer using a surface acoustic wave (hereinafter referred to as SAW) which is suitable for such a mobile radio terminal.
2. Background Art
An antenna duplexer is configured such that a transmitter-line filter 2 and a receiver-line filter 3 are connected in parallel to each other through an antenna terminal 1 as a common terminal, as shown in FIG. 1. A background-art antenna duplexer was constituted by using transmitter-line and receiver-line filters in which a large number of coaxial dielectric resonators were connected in cascade, for example, as described in xe2x80x9cHow dielectric filter technologies respond to requirements of future communication systemsxe2x80x9d, 1998 Asia-Pacific Microwave Conference Proceedings, pp.1445-1452.
The mobile communication represented by portable telephone commenced its service in an analog system for transmitting: only voice. After that, a digital system which could also transmit data started and has been continuing up to the present in some locations. In addition, to these systems, a CDMA (Code Division Multiple Access) system as well as these systems has been already introduced. The CDMA system has superior features such as message screening from eavesdrop, soft hand over, and so on, and it is said that the CDMA system will account for about half of all the mobile communications in the middle of the 21th century.
FIG. 2 is a block diagram of a mobile radio terminal. An antenna duplexer 4 is configured such that a transmitter-line filter 2 and a receiver-line filter 3 are connected in parallel to each other through an antenna terminal 1 as a common terminal, as shown in FIG. 1. With this configuration, the antenna duplexer separates very high transmitted power (usually 0.2 W to 1 W) in a transmitter band from a transmitter (here designating a portion from a modulator (Mod.) to a high power amplifier (HPA) 7) and very weak received power (usually about xe2x88x92100 dB to xe2x88x92150 dB of the transmitted power) in a receiver band coming in through an antenna, and sends the former to the antenna and receives the latter to the receiver (here designating a portion from a low noise amplifier (LNA) 8 to a demodulator (Demod.)).
One of large differences between the digital system and particularly the CDMA system is that transmission and reception are not performed simultaneously in the digital system because a burst-like. transmission signal and a burst-like reception signal are shifted from each other on the time base while transmission and reception are always performed simultaneously in the CDMA system. In the digital system, therefore, even if a part of transmitted power wanders into a receiver side, there is little influence on the characteristics of the receiver as long as an LNA of the receiver is not broken.
In the CDMA system, on the other hand, transmission and reception are performed simultaneously. It is therefore necessary for a receiver-line filter of an antenna duplexer to suppress transmitted power satisfactorily so as to sufficiently restrain the transmitted power from entering a receiver side. That is, the receiver-line filter should have a large quantity of attenuation in the transmitter band. In addition, noise generated from an HPA in the receiver band and superposed on received weak power from an antenna is put into the receiver. The sensitivity characteristics of the receiver deteriorate thus. It is therefore necessary for a transmitter-line filter of the antenna duplexer to suppress the receiver band noise from the HPA satisfactorily. That is, the transmitter-line filter should have a large quantity of attenuation in the receiver band.
Generally, in transmitter-line and receiver-line filters of an antenna duplexer, as the bandwidth of each of the transmitter and receiver bands is broader, and as the space (guard band) between the transmitter and receiver bands is narrower, it is more difficult to attain frequency characteristics required of the filter. In recent portable telephones, there is a tendency to make the bandwidth of each of the transmitter and receiver bands as wide as possible so as to ensure a large number of talk channels, and to make the guard band between the transmitter and receiver bands as narrow as possible so as to use the frequency effectively. Such a tendency is generally conspicuous particularly in the latest systems such as portable telephones using CDMA.
FIGS. 3A and 3B show the frequency allocation (FIG. 3A) of J-CDMA (!Japan CDMA) served for CDMA portable telephones in Japan and the frequency allocation (FIG. 3B) of PCS (Personal Communication System) served for CDMA portable telephones in the U.S, by way of example. In J-CDMA, the guard band is 17 MHz, and each of the transmitter and receiver bandwidths is 38 MHz including a null frequency bandwidth of 14 MHz. How wide each of the transmitter and receiver bandwidths is and how narrow the guard band is in this frequency allocation will be understood easily from the following comparative example. For example, in the frequency layout of well-known 800 MHz band portable telephone AMPS (Advanced Mobile Phone Service) in the U.S., the transmitter bandwidth is 25 MHz, that is, from 824 MHz to 849 MHz, the receiver bandwidth is also 25 MHz, that is, from 869 MHz, to 894 MHz, and the guard band is 20 MHz. In comparison therewith, in J-CDMA, each bandwidth increases by 52% and the guard band decreases by 18%.
In PCS: which uses a 1.9 GHz band, each of the transmission and receiver bandwidths is 60 MHz, and the guard band is 20 MHz. When the center frequency of PCS is scaled to the center frequency of AMPS, each bandwidth increases by 7% and the guard band decreases by 55%.
As has been described, particularly in a recent antenna duplexer for use in a radio terminal for CDMA or the like, severe requests are laid on the frequency characteristics of transmitter-line and receiver-line filters because transmission and reception are performed simultaneously. In addition, there is a request to broaden each of the transmitter and receiver bandwidths and to narrow the guard band. Thus, the fact is that it is very difficult to realize an antenna duplexer having properties satisfying all the requests.
In a background-art filter configuration in which a large number of coaxial dielectric resonators are connected in cascade, the number of stages of the dielectric resonators is increased to cope with such difficulty. However, the increase in the number of the stages results in increase of the volume of the antenna duplexer itself. As a result, the antenna duplexer becomes a largest and heaviest device in a recent CDMA terminal so as to place hurdles on the miniaturization of the terminal as a whole.
It is therefore an object of the present invention to solve the foregoing problems.
It is another object of the present invention to provide an antenna duplexer which is micro-miniaturized and reduced in weight, and hence to contribute to the miniaturization of a radio terminal as a whole.
In order to attain the foregoing objects, at least one of transmitter-line and receiver-line filters of an antenna duplexer is constituted by two partial filters connected in cascade. Of the partial filters, the antenna terminal side filter is referred to as a top filter. A surface acoustic wave (SAW) resonator filter is used as the transmitter-line or receiver-line terminal side partial filter. The top filter is required to have a frequency characteristics similar to those of an antenna duplexer used in a digital-system radio terminal. That is, the quantity of attenuation in each of the transmitter and receiver lines may be sufficiently small in the other""s band
It is known that an SAW resonator filter can realize a very low loss in the pass band, that is, allow-loss characteristic, when a narrow-band filter is made of the SAW resonator filter. In addition; the SAW resonator filter has another advantageous characteristics that a very large quantity of attenuation can be attained: at a specific frequency. We have found that if an SAW resonator filter is constituted by a combination of at least one SAW resonator and at least one additive capacitance or inductance, and if the value of the capacitance or inductance is varied by a switching element, the frequency of the pass band or the frequency of the attenuation band can be varied. This means that band switching can be attained by the SAW resonator filter.
Generally, SAW filters or SAW resonators can be implemented in the form of chips in the same manner as semiconductor ICs so as to be extremely small both in volume and in weight. Therefore, even including such additive capacitance or inductance and further including such a switching element and so on, the above-mentioned band-switchable SAW resonator filter can be miniaturized sufficiently.
As has been described, according to the present invention, a transmitter- line or receiver- line filter of an antenna duplexer is constituted by a top filter and a band-switchable SAW resonator filter which are connected in cascade so as to cover all the transmitter,or receiver bandwidth equivalently through changing over the band by switching and, at the same time, so as to such ensure a required quantity of attenuation by the composition of the top filter and the SAW resonator filter. Here, the top filter may be a background-art filter in which a large number of coaxial dielectric resonators are connected in cascade, or may be an SAW filter that has performance equal to that of the dielectric resonator filter and that is disclosed for example in xe2x80x9cSAW antenna duplexer module using SAW-resonator-coupled filter for PCN systemsxe2x80x9d, 1998 IEEE Ultrasonics Symposium Proceedings, pp.13-16. As a result of linear calculation, it was found that the volume could be made not larger than xc2xd of that of the background-art when each of the transmitter-line and receiver-line filters was constituted by cascade connection of a dielectric resonator filter as a top filter and a band-switchable SAW resonator filter according to the present invention. In addition, it was found that the volume could be made to be one to several of that in the background-art when each of the transmitter-line and receiver-line filters was constitutedby cascade connection of an SAW filter as a top filter, which had performance similar to the dielectric resonator filter, and a band-switchable SAW resonator filter according to the present invention.